The exhaust gas discharged from an engine contains, inter alia, hydrocarbon (HC), carbon monoxide (CO), nitrogen oxides (NOx), and particulate matter (PM). In order to purify these exhaust gas components, an exhaust gas purification catalyst is disposed in an automobile, for example, immediately downstream from the engine exhaust manifold.
In general, when the exhaust gas temperature is still low, such as at engine start, the exhaust gas purification catalyst is not adequately warmed up and the purification performance by the catalyst is then reduced. In addition, the low-temperature exhaust gas contains large amounts of CO and HC that is uncombusted material from the fuel. As a consequence, an unsatisfactory purification of the HC and CO by the exhaust gas purification catalyst has been a problem when the exhaust gas resides at low temperatures. HC and CO are converted to H2O and CO2 by oxidation for discharge. Thus, there is demand for an improved low-temperature activity by exhaust gas purification catalysts, particularly with regard to the oxidative purification performance.
Various attempts have been made to improve the purification performance for HC and CO. As one such attempt, an exhaust gas purification catalyst is known that contains zeolite as a material that adsorbs the HC component.
Patent Literature 1 discloses a diesel oxidation catalyst that contains a first part and a second part: a hydrocarbon trap that adsorbs or desorbs hydrocarbon is coated in the first part, and an oxidation catalyst that oxidizes hydrocarbon and carbon monoxide is coated in the second part. This second part oxidizes HC desorbed from the first part and, utilizing the heat of oxidation produced by this oxidation reaction, desorbs sulfur that has been adsorbed by the oxidation catalyst. This brings about an improved catalyst activity by raising the catalyst bed temperature through the generation of the heat of oxidation utilizing the adsorbed and desorbed HC component.
Other exhaust gas purification catalysts that contain a component that adsorbs the HC component are disclosed in Patent Literature 2 to 7.